A continuing challenge of achieving nanoscale integration in the semiconductor industry is to fabricate an ever-increasing number of devices, e.g., transistors, into an ever-decreasing area by shrinking device footprint, while retaining high yields and reliability. This has been achieved for digital applications mainly by scaling down transistor channel lengths while managing associated short-channel effects, e.g., a decrease of threshold voltage (Vt) due to charge sharing between the gate and source/drain diffusion regions. This has lead to reduced-thickness gate insulator, e.g., oxide, layers, increased channel-doping concentrations, and use of various three-dimensional structure construction techniques.
In previous approaches, transistors have been fabricated in a horizontal plane. One previous approach to address the above-mentioned nanoscale challenge is to construct transistors on a vertical plane, essentially orienting them on edge to reduce the real estate required per transistor in the horizontal plane, for example, forming field-effect transistors (FETs) on a thin vertical semiconductor layer, e.g., silicon (Si) provided in a thin wall-like fashion, e.g., a Fin, having a thin insulating film. Multiple gates can be formed, e.g., one on each of two sides, of the free-standing, vertically-oriented Fin, with the gates being electrically connected to cooperatively modulate the channel therebetween. FETs formed on these Fin-like structures are referred to as Fin-FETs or multi-gate FET structures (MuGFETs).
The fundamental configuration of a metal-oxide-semiconductor (MOS) FET, i.e., MOSFET, and a junction FET, i.e., JFET, are well known. A MOSFET gate controls current by application of a voltage to the gate, causing a field effect in the surface of the semiconductor, and either a buildup or depletion of charge in the wafer surface under the gate, depending on the doping conductivity type in the wafer under the gate and the polarity of the gate voltage. This buildup or depletion of charge creates an electrically-conducting channel under the gate, which connects a source region to a drain region. An n-channel MOS transistor, i.e., NMOS, has n-type source and drain regions formed in a p-type wafer. When a positive gate-source voltage, VGS, is applied, an n-channel is created at the surface of the p-type region, just under the insulating layer, by depleting the channel region of holes and attracting electrons to the surface. Conversely, a p-channel MOS transistor, i.e., PMOS, has p-type source and drain regions formed in an n-type body. A p-channel is created at the surface of the n-type channel region when a negative gate to source voltage, e.g., VGS, is applied, depleting the channel region of electrons and attracting holes to the surface.
A JFET configuration has a junction formed under a gate conductor. As is the case with MOSFETs, JFETS can be formed having an n-channel, i.e., nJFET, or p-channel, i.e., pJFET. Furthermore, JFETS can be fabricated to operate as a depletion mode device (normally-on and conducting at zero gate voltage, turned-off by applying a reverse bias voltage on the gate having a magnitude in the range of approximately 1.2 Volts to 1.8 Volts, negative for an nJFET and positive for a pJFET) or an enhancement mode device (normally-off at zero gate voltage, turned-on by application of a small forward bias voltage on the gate, positive for an nJFET and negative for a pJFET).
Rather than an insulated gate, a field is applied by the junction acting as a gate. During operation, e.g., of an nJFET, current flows from the source to the drain in a doped silicon region under the gate. As the nJFET gate voltage is increased, a region depleted of charge (the depletion region) spreads, pinching-off the conducting path. Due to the lack of available mobile charge, the depleted region behaves like an insulator, and thus has the effect of pinching-off, i.e., restricting, current as it increases in depth. An nJFET can operate opposite from an enhancement-mode MOSFET. In an enhancement-mode nMOSFET, increasing the gate voltage increases current through the channel. However, in an nJFET, increasing the gate voltage decreases current through the channel.
Complementary MOS logic, i.e., CMOS, uses p- and n-channel MOSFETs as building blocks. In an inverter, each n-type MOSFET, i.e., nMOSFET, is complemented with a p-type MOSFET, i.e., pMOSFET, by connecting both gates and drains together. A high voltage on the gates will cause the nMOSFET to conduct and the pMOSFET not to conduct. A low voltage on the gate causes the reverse conductivity. MOSFETs are used primarily in digital, memory, and analog circuit applications. However, JFETs are known to better meet analog requirements calling for analog components having high signal-to-noise ratio, low parameter mismatch, linearity, and low flicker noise.
The development of Fin-MOSFETs to date has emphasized performance and density for low voltage digital and memory applications. In particular, a focus has been placed on the reduction of short-channel effect, sub-threshold slope (associated with reduced threshold voltage and inability to completely turn-off a transistor), and effective area for a given channel width. Several non-planar structures have been demonstrated in previous approaches of forming Fin-MOSFETS. Initially, the MOSFET channel was formed on a silicon trench sidewall. Thereafter, a “surround-gate” vertical MOSFET was constructed using a directional etch technique to form a silicon pillar, with the pillar surrounded by a dielectric and having a source on the top of the pillar, a drain on the bottom, and a gate in between. In another previous approach, a lateral Fin-MOSFET was demonstrated using a silicon-on-insulator (SOI) configuration, whereby the SOI was formed by lateral oxidation under a single-crystal silicon pillar, thus separating the pillar from the supporting wafer, with a source, channel, and drain being formed along the Fin. Other variants of Fin-MOSFET structures have been fabricated with varying degrees of performance and complexity.